1. Technical Field
The present invention relates to image forming apparatus that forms an image with dry toner (developing powders), such as printers, copiers, facsimiles, and hybrid machines as well as a method of recommending replacement of a rotatable member, a method of cleaning a rotatable member, and a method of controlling image formation. Particularly, the invention relates to an image forming apparatus provided with a new function for preventing toner offset from occurring in a fusing device that fuses and fixes a toner image transferred onto a sheet of paper as well as a method of recommending replacement of a rotatable member, a method of cleaning a rotatable member, and a method of controlling image formation.
2. Related Art
An image forming apparatus such as printers applying an electrophotographic method or an electrostatic recording method generally forms an image with dry toner on a sheet of paper in the following way.
First, based on image data, a toner image is formed on an image carrying member such as a photo conductor through a predetermined imaging process (including charging, exposure, and development steps). The toner image is transferred to a sheet of certain paper directly or via an intermediate transferring member. Then, by exerting heat and pressure on the sheet having the toner image transferred thereto, a fusing device thermally fixes. the toner image to the sheet. As the fusing device, such a type is widely used at the present that employs a pair of rotatable members for image fixing (e.g., a heating roller and a pressure roller) that defines a contact pressing portion that heats and presses the sheet with the toner image transferred thereto, guided to pass through it, thereby fusing and fixing the toner image to the sheet.
In such image forming apparatus, it is practiced to form a release layer made of a fluorinated resin or the like on the surfaces of the rotatable members for image fixing in the fusing device to improve the releasability of the rotatable members. However, there may occur a phenomenon, which is so-called toner offset, in which some of toner not yet fixed to the sheet transfers and clings to the surfaces of the rotatable members for image fixing with the release layer formed thereon. If the toner offset occurs in the fusing device, the toner offset to the rotatable members for image fixing would be deposited onto a sheet passing through the contact pressing portion for subsequent image fixing. This poses problems such as a stain on a sheet and a blurred part of a fixed image resulting in a defective image.
As measures to eliminate the problems associated with toner offset occurrence, it has heretofore been carried out that a cleaning device is installed at the rotatable members for image fixing to remove the toner offset to the rotatable members. However, in a case where a type of paper (such as acid-free paper) including calcium carbonate as a loading filler, for which demand has grown recently, is used, the calcium carbonate particles adhere to the rotatable members for image fixing. This poses new programs such as a decrease in the releasability and toner offset occurring due to the calcium carbonate particles adhered to the rotatable members.